1. Field of the Invention
This invention relates to a wave motor using ultrasonic oscillation to drive a moving body, and more particularly to a travelling-wave motor which drives a moving body frictionally by travelling-wave generated by a applying electric signals to an electromechanical transducer adhered to an elastic body.
2. Description of the Prior Art
Two types of the ultrasonic motor are conventionally known. One is a standing-wave type motor which employs a Langevin oscillator as a driving source. Such a type of motor is disclosed in U.S. Pat. No. 4,019,073. The other is a travelling-wave type motor employing a travelling wave generated on a stator for driving a rotor provided on the stator. The travelling-wave motor generates smaller wear in the friction-transmition surface between the stator and the rotor, and is easier to drive in the reverse-rotation direction in comparison with the standing-wave type motor. Such a travelling-wave motor is disclosed in U.S. Pat. No. 4,513,219 to Katsuma et al., U.S. Pat. No. 4,562,374 to Sashida and European Pat. Appl. Pub. No. 169297 of Tokushima. Katsuma et al. and Sashida disclose travelling-wave motors employing a ring type of a piezoelectric member. This type of travelling-wave motor essentially consists of an annular vibrating body and a movable body provided thereon. The vibrating body has an annular piezoelectric vibrator thereon.
Another type of travelling-wave motor employing a disk-shaped piezoelectric member is disclosed in the European Patent Application of Tokushima. In such a motor, a stator is a disk-shaped elastic vibrating body having a toothlike circular protrusion. The vibrating body has a disk-shaped piezoelectric vibrator thereon. A movable body is provided on the protrusion of the vibrating body and has a central shaft for rotational guide. A pressure-regulating mechanism is provided on the central shaft for making a suitable contact pressure between the vibrating body and the movable body to efficiently conduct the travelling wave component to the movable body.
Since the travelling-wave motors employ friction between the vibrating body and movable body to drive the movable body, the rotating velocity changes according to the condition of the frictionally engaged surfaces. Moreover, the rotating velocity also changes depending on a voltage applied to the piezoelctric element. Travelling-wave motors employing mechanisms for controlling the rotating velocity and rotational position of the movable body are disclosed in Japanese Laid Open Patent Nos. sho 60-51478 and sho 60-113675. The travelling-wave motor disclosed in the Laid Open Patent No. sho 60-51478 employs a divided metal pattern formed on the upper surface of the movable body and a rotation detecting leaf spring pushing the upper surface of the movable body in order to derive rotational position information as electric signals. While the travelling-wave motor disclosed in the Laid Open Patent No. sho 60-113675 employs a photo diode, a through hole provided in a disk-shape movable body and a photo transistor in order to derive rotational position information as electric signals. Both conventional travelling motors described above conduct detection of position every time in accordance with a drive of the wave motor. Therefore, the travelling-wave motors described above have a shortcoming that much energy is consumed for detection of the position of the movable body.